1. Field of the Invention
The invention relates to an apparatus for measuring the downhole force applied to a cable suspending a well-logging apparatus in a borehole.
2. Description of the Prior Art
To accurately determine the depth of a well-logging tool suspended at the end of a cable in a borehole, it is necessary to measure the displacement of the cable on the surface and to correct the surface displacement measurement by the calculated elongation of the cable. The elongation calculation requires information relating to the surface and downhole tensions applied to the cable. As a first approximation, it is possible to take, as a downhole force, the apparent weight of the apparatus in the column of drilling mud. However, for apparatus equipped with pads or centering devices, or for directionally deviated wells, the downhole force depends greatly upon the friction of the apparatus against the borehole wall. Accordingly, it is desirable to measure the downhole force in situ by means of a cable force measuring device placed between the cable and the well-logging apparatus.
U.S. Pat. No. 3,497,958, issued to Gollwitzer on Mar. 3, 1979, and U.S. Pat. No. 3,402,601, to Heineman, issued on Sept. 24, 1968, illustrate two devices for measuring downhole tension. Gollwitzer describes a method for determining the depth of a well-logging apparatus using a measurement of movement of the cable and the values of the downhole tension of the cable. Gollwitzer also schematically describes a device for downhole tension measurement using strain gauges; however, neither the specific structure of the downhole tension device, nor the means by which the strain gauge elements are mounted is disclosed. The Heineman patent discloses a tension-measuring sensor mounted in an oil-filled enclosure in a downhole tool head. In order to correctly carry out the measurement of downhole tension, Heineman discloses the use of a bellows-type of device for providing pressure and temperature compensation against the difficult temperature and high pressure conditions encountered in the borehole, which can be as high as temperatures of 500.degree. F., and pressures of 25,000 p.s.i. in the drilling mud column.
One of the major problems encountered when attempting to measure the downhole force applied to a cable suspending a well-logging apparatus is that when a bellows-type temperature pressure compensation means is utilized, the drilling mud may become entrapped in the creases of the bellows. As the bellows expands and contracts in relationship to pressure and temperature changes, the drilling mud may prevent the necessary contraction of the bellows, or may cause a failure of the metal bellows by causing it to crack, rip open, or corrode. Another deficiency with the bellows-type pressure and temperature compensation means is that it is inefficient from a space standpoint, in that additional space in the tool head is required for mounting the bellows-type pressure and temperature compensation device.
A major problem encountered when using strain gauge devices in downhole cable tension measuring apparatus is related to the extreme temperature and pressure conditions encountered in the boreholes. Heretofore, strain gauges utilized in downhole cable tension measuring apparatus were able to operate under high temperature or under high pressure conditions; however, when both high temperature and pressure conditions were encountered, the accuracy of the tension measuring device was adversely affected, including unacceptable levels of drift and instability of the strain gauge signals.
Accordingly, prior to the development of the present invention, there has been no downhole cable force measuring apparatus which has satisfactory operating characteristics when operating under extreme temperature and pressure conditions, and which is provided with a space-saving pressure and temperature compensation means whose operation is not adversely affected by coming into contact with the column of drilling mud. Therefore, the art has sought an efficient and accurate downhole cable force measuring apparatus which provides accurate cable force measurements under high temperature and pressure conditions, and is not adversely affected by the column of drilling mud coming into contact with the pressure and temperature compensation means.